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Minimum Exit Pupil?

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JAM
JAM's picture
Minimum Exit Pupil?

The visual observing manual mentions (in passing) challenges using less than a 2mm exit pupil, and also indicates E and F chart scales are for large scopes.

I'd intended to use my 4" refractor with exit pupils down to 0.5mm, and E and F charts for fainter variables, particularly at home in brighter skies.  

I don't understand if the above is a hindrance to variable observation (or if the manual indeed implies that). I thoroughly understand the concept of exit pupil/magnification as it relates to contrast/limiting magnitude etc.

I'm keen to submit accurate data, so if any experienced observers see an issue with this I am happy to hear from you.

Thanks,
Andrew

pox
pox's picture
As a general rule the size of

As a general rule the size of the observer's eye pupil decreases with age, from a maximum of (I believe) about 7mm, so for example 7 x 50 binoculars which give this size exit pupil make optimum use of your 'organic detectors' as does any optical system giving a comparable exit pupil. Small EPs present unfortunately a host of attendant problems. I know when I use a high power (not very often if possible) it is physically a challenge to keep the small exit pupil from falling on the centre of my eye pupil.

JAM
JAM's picture
Thanks for your response.

Thanks for your response.

If averted vision with small exit and eye pupils is the main difficulty, that may not apply to me; my eye pupil is 7-8mm. I don't match an exit pupil to that unless I'm optimizing surface brightness for extended objects, or (incidentally) when lowering magnification for bright variables. I love low power views, but not at the expense of a broad selection of fainter targets I could otherwise accurately observe.

Please feel free to let me know if I've missed or misunderstood something, though.

tst
Exit Pupil

Hi Andrew, 

Please correct me if I'm wrong, but I believe the context of the 2mm exit pupil is if the telescope  system  provides an exit pupil of 7mm but the  observer's has a maximum exit pupil of 5mm, then 2mm will be wasted. The affect would be the same as stopping down the objective.

Personally, for my 6" F/8 Newtonian I find, exit pupils of 3mm, 2mm, and 1mm to be my most used.

A 3mm exit pupil is the largest I can go (50' fov @ 50X) before the telescope's tube starts to cut into the light path (vignetting) and 1mm is as small as I can go before floaters in my eye become a distraction. 

I hope this helps.

Regards 

Steve Toothman TST 

   

pox
pox's picture
confusing advice re charts

I find that advice on charts a bit misleading. Surely the point about E and F charts is the scale, rather than the faintness of the stars. It is true that they do include fainter stars but that is more a consequence of 'zooming in' to a smaller area. An example: several years ago when I was drawing up 'my' charts I had to draw E and/or F charts for at least 3 variables  (AD Aql, BO Cep and V561 Cyg)  even though none of them are excessively faint - but they do all have close companions which therefore necessitates E or F scale charts. Indeed the very notation of the 'lettered' charts derives from the scale factor, not the faintness of the stars, though this is part of the picture. That's why the chart plotter is so great because you can have (say) an F chart which only shows stars down to (say again) 13th mag. if that's all that is required!

JAM
JAM's picture
You're correct about the chart scales.

Perhaps the manual could be tweaked for clarity. E and F charts require <1mm exit pupils in my case, but larger scopes (generally) don't have this issue so that's how I interpreted their statements.

I also set limiting magnitudes for each scale to ensure I use the right equipment, 'zooming in' to the next chart/eyepiece when the comp stars are too bright… I just assumed that's how most folks did it!

tst
Off Topic

POX,

Thanks for the great idea. When making estimates with the 8X50 finder I normally use "b" charts and "d" charts with my 6"  newtonian.  The "b" and "d" charts show stars one or two magnitudes fainter than the associated scope. Changing the limiting magnitude of the charts to match the limiting magnitude of the two scopes, cleans up the clutter and makes it easier to identity the target.

Thanks 

Steve Toothman TST 

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